The xylenes, para-xylene, meta-xylene and ortho-xylene, are important intermediates which find wide and varied application in chemical syntheses. Para-xylene upon oxidation yields terephthalic acid, which is used in the manufacture of synthetic textile fibers and resins. Meta-xylene is used in the manufacture of plasticizers, azo dyes, wood preservers, etc. Ortho-xylene is feedstock for phthalic anhydride production.
Xylene isomers from catalytic reforming or other sources generally do not match demand proportions as chemical intermediates, and further comprise ethylbenzene, which is difficult to separate or to convert. Para-xylene in particular is a major chemical intermediate with rapidly growing demand, but amounts to only 20–25% of a typical C8 aromatics stream.
Ethylbenzene generally is present in xylene mixtures and is occasionally recovered for styrene production, but usually is considered a less-desirable component of C8 aromatics.
Among the aromatic hydrocarbons, the overall importance of the xylenes rivals that of benzene as a feedstock for industrial chemicals. Neither the xylenes nor benzene are produced from petroleum by the reforming of naphtha in sufficient volume to meet demand, and conversion of other hydrocarbons is necessary to increase the yield of xylenes and benzene. Often toluene (C7) is dealkylated to produce benzene (C6) or selectively disproportionated to yield benzene and C8 aromatics from which the individual xylene isomers are recovered.
A current objective of many aromatics complexes is to increase the yield of xylenes and to de-emphasize benzene production. Demand is growing faster for xylene derivatives than for benzene derivatives. Refinery modifications are being effected to reduce the benzene content of gasoline in industrialized countries, which will increase the supply of benzene available to meet demand. Benzene produced from disproportionation processes often is not sufficiently pure to be competitive in the market. A higher yield of xylenes at the expense of benzene thus is a favorable objective, and processes to transalkylate C9 aromatics and toluene have been commercialized to obtain high xylene yields.
U.S. Pat. No. 4,857,666 (Barger et al.) discloses a transalkylation process over mordenite and suggests modifying the mordenite by steam deactivation or incorporating a metal modifier into the catalyst.
U.S. Pat. No. 5,043,509 (Imai et al.) discloses a process for conversion of organic compounds with a phosphoric catalyst consisting of a shaped extrudate with a specific maximum ratio of length to diameter representing the ratio of exterior surface area to catalyst volume. Shaped catalysts have also been described in U.S. Pat. No. 4,185,040 (Ward et al.) for use in an olefin alkylation catalyst comprising Y zeolite.
U.S. Pat. No. 5,763,720 (Buchanan et al.), discloses a transalkylation process for conversion of C9+ over a catalyst containing zeolites illustrated in an extensive list including amorphous silica-alumina, MCM-22, ZSM-12, and zeolite beta, where the catalyst further contains a Group VIII metal such as platinum.
U.S. Pat. No. 5,942,651 (Beech, Jr. et al.) discloses a transalkylation process in the presence of two zeolite containing catalysts. The first zeolite is selected from the group consisting of MCM-22, PSH-3, SSZ-25, ZSM-12, and zeolite beta. The second zeolite contains ZSM-5, and is used to reduce the level of saturate co-boilers in making a higher purity benzene product.
U.S. Pat. No. 5,952,536 (Nacamuli et al.) discloses a transalkylation process using a catalyst comprising a zeolite selected from the group consisting of SSZ-26, A1-SSZ-33, CIT-1, SSZ-35, and SSZ-44. The catalyst also comprises a mild hydrogenation metal such as nickel or palladium, and can be used to convert aromatics with at least one alkyl group including benzene.
U.S. Pat. No. 6,060,417 (Kato et al.), discloses a transalkylation process using a catalyst based on mordenite with a particular zeolitic particle diameter and having a feedstream limited to a specific amount of ethyl containing heavy aromatics. Said catalyst contains nickel or rhenium metal.
Thus, many types of supports and elements have been disclosed for use as catalysts in processes to transalkylate various types of aromatics into xylenes. However, applicants have found that the specific size of the catalyst used for these processes provides a surprising benefit when reduced to a smaller than expected size, or shaped to increase the exterior surface area per gram of catalyst. Both of these size limitations effectively increase the diffusion of C6 to C10 aromatics and enhance aromatic mass transfer rates to the surface.